The use of diamond-like carbon (DLC) coatings is known in the medical industry as a means to decrease the frictional wear of metallic components. DLC coatings have been used, for example, on articulating components of medical devices, e.g., hip replacements, to reduce surface wear. In these devices, the DLC-coated component typically articulates against a polymeric or DLC-coated counterpart. For example, a total disc replacement device for the spine may have a DLC-coated titanium alloy component that articulates against a polyethylene counterpart.
DLC coatings applied directly on to a substrate may, however, demonstrate poor adhesion stability. Due to the deposition mechanism, DLC coatings can possess excessive compressive stress in the GPa range, which favors delamination of the DLC coating from a substrate. For example, published data on certain currently available DLC-coated hip joints exhibit massive failures after 9 years in vivo.
FIG. 6 shows the revision rates of certain DLC-coated hip joint implants according to the prior art gained from a 101 implants study by Taeger et al. (Materialwissenschaften und Werkstofftechnik 2003; 34(12): 1094-1100, incorporated herein by reference in its entirety). FIG. 7 shows a hip joint head explant from the Taeger series. As can be seen, the DLC-coating has failed and caused significant wear. The origin of the failures is small delaminated spots on the DLC surface, which eventually combined to give one massive failure. Upon closer inspection, the failures appear roughly circular and can be shown to originate from a small point of failure, probably a pinhole as shown in FIG. 8. Delamination occurred in a circular fashion from an initial spot in the center.
FIG. 9 shows the delamination of the Taeger coating system originating from an artificial defect. The delamination speed rapidly increases after 240 days, at which time the storage medium was exchanged from phosphate buffered saline (PBS) to calf serum. The other data points give the energy sustained by and stored in the coating system.
Thus, there remains a need for an improved DLC coating with improved long-term adhesion in vivo.